The present invention relates to a device for the decimation of data sequences representing digital samples of a signal to be subsampled, the successive samples of which are represented by the successive matrix elements of a matrix with N rows and M columns.
Decimation is an operation which consists in reducing the size of a data block and relies on low-pass type filtering followed by sub-sampling.
The data block processed can be two-dimensional, such as a digital image containing N/2 even lines constituting an even frame and N/2 odd lines constituting an odd frame. In this case, decimation consists in reducing the size and resolution of the image processed. One of the most often used techniques consists in replacing a series of neighbouring points of the image by their mean. The decimation factor defines the mode of decimation and corresponds to the number of points averaged and replaced by a single point.
The devices used in the prior art to perform this operation generally include several calculation stages mounted in cascade, each stage periodically averaging the data which it receives from the preceding stage during a calculation cycle of at least one clock period.
Such devices require a large number of operators contributing to the rise in the cost of their manufacture.
Represented diagrammatically in FIG. 1 is a structure illustrating a decimation device of the prior art containing three stages 10, 12 and 14 mounted in cascade. Each of the three stages 10, 12 and 14 includes an adder 16 with two inputs 18 and 20, the input 18 receiving the data to be decimated while the input 20 receives the said data across a delay operator (22, 24, 26). The said adder 16 being connected via a sampling device 30, to the inputs 18 and 20 of the adder of the following stage.
Apart from the large number of operators which they contain, the decimation devices of this type do not allow optimum use of the various calculation stages. Indeed, the operations carried out by these devices are done sequentially over several successive calculation cycles in the course of which they produce alternately a useful result representing the mean, obtained in the course of a first calculation cycle, of a first sample P(i,j) and of a second sample P(i,j+1), and a non-useful result representing the mean, obtained in the course of the following calculation cycle, of the sample P(i,j+1) and of the sample P(i,j+2). Now, only the useful result is taken into account in the decimation operation. Such functioning is illustrated by FIG. 1a, representing for example sixteen pixels P(1,1) to P(1,16) situated in a line L, of a digital image. In this case, the decimation of the digital data representing the said pixels consists in replacing two adjacent pixels by a single pixel. For this purpose, the stage 10 of the device of FIG. 1 carries out in succession the calculation of the mean MP1 of the digital data representing the pixels P(1,1) and P(1,2), then that of the mean MP2 corresponding to the pixels P(1,2) and P(1,3), then that of the mean MP3 corresponding to the pixels P(1,3) and P(1,4), then that of the mean MP4 corresponding to the pixels P(1,4) and P(1,5), then that of the mean MP5 corresponding to the pixels P(1,5) and P(1,6) and so on up to the mean MP15 corresponding to the pixels P(1,15) and P(1,16). As may be observed, the stage 10 makes it possible to calculate fifteen different means MP1 to MP15 of which only the means bearing an odd index are useful, stated otherwise, the stage 10 produces only 50% of useful results in the course of a calculation cycle. Indeed, each of the means bearing an even index MP2, MP4, MP6 . . . MP14 involves the digital data corresponding to the pixel P(1,2), P(1,4), P(1,6) . . . P(1,14), which data are already used in the calculation of the means bearing an odd index MP1, MP3, MP5 . . . MP15 and which represent the useful results. The stage 12 then makes it possible to calculate the successive means of the digital data representing the pixels obtained by the calculation of the stage 10 and produces 50% of useful results on the basis of the previously calculated results, namely only 25% of useful results, obtained on the basis of the data received by the stage 10. Similarly, stage 14 will produce 50% of useful results on the basis of the 25% of the results received, namely 12.5% of useful results obtained on the basis of the data received by the stage 10.